If a US recipe specifies 'vegetable oil' what type of oil should I use in the UK?

Peanut (groundnut) oil is a great option.

In the US vegetable oil generally means soybean oil or a soybean oil blend. The main things are that it be neutral (little or no taste of its own), with a high smoke point. On those scores, you can't do much better than peanut oil.

I have not used rice bran oil.

Yellow squash generally means this:

(the long one)

It can also mean the other yellow vegetable in the picture or this:

What it is not, is butternut:

That is a different flavor entirely. Yellow squash is more like zucchini, butternut squash is more like pumpkin.

Welcome to Seasoned Advice.

I looked to the wiki I've been maintaining on translating between English dialects, but I realized that the 'yellow squash' distinction was a bit muddled in there:

• Summer Squash (US) are members of the squash family with a short storage life typically harvested before full maturity; typically available starting in the spring and summer; includes zucchini, yellow and crookneck squash.
• Winter Squash (US) are members of the squash family that are allowed to reach full maturity before harvesting; typically available in the fall; includes pumpkin, acorn and butternut squash.

I wouldn't substitute butternut squash, as it's a much sweeter, firmer variety. You want something that's an immature squash -- it still has thin, edible skin and hasn't yet formed a distinctive seed cavity.

As Jolenealaska has mentioned, zucchini (courgette in the UK) will do, but if you can find it, a pattypan squash might be a bit closer.

In Canada "Vegetable Oil" is generally taken as 100% unblended canola/rapeseed oil

Refined canola oil has a smoke point of 400F, according to: http://www.goodeatsfanpage.com/collectedinfo/oilsmokepoints.htm

You can choose from that chart any of the oils that fit your temperature range, and provide the degree of flavouring you desire.

In Britain the two most common vegetable oils are Sunflower oil and Rapeseed Oil. Sunflower oil has a smoke point of over 400F, and Rapeseed oil similar, assuming both are refined which is almost always the case as sold in supermarkets. Rapeseed is reported to have higher omega-3s than Sunflower oil so is increasingly popular, but Sunflower oil is very commonly used.

"Vegetable oil", in the US, generally means canola oil (aka rapeseed oil).

(In many cases, of course, you could substitute another neutral oil with a similar smoke point, if canola isn't available.)

In the U.S. Yellow Squash is the second one pictured. It is a staple here during the summer months. Also tasty sautéed with Vidalia or sweet onions.